@Account Closed notes above you can mail merger from Outlook contacts. If your list is in Excel you can merge from that as well, which is what I do. Here's a tip, in case any of your mail address Zip Codes start with a "0" save the excel file as a .csv and format the zip code column as Special:Zip Code. For some reason when saving as an .xlsx even if the column is correctly formatted as soon as you merge into word it drops the leading "0"
Template: I made a custom MS Word template for the specific lined paper I buy, with the correct margins, line spacing and font so all I do is open up a document, and instead of choosing blank document I choose my custom template so everything will line up perfectly.
Envelope: I use invitation envelopes, they're 8.5 x 5.5 so the 8.5 x 11 paper fits folded in half. Note that I also have an MS Word template for my specific envelopes as well.
Alternatively you can outsource the whole thing. I am the type that prefers to outsource everything I can, but I haven't been able to justify outsourcing this yet because I've got it set up so that it takes care of itself while I'm doing other things. The worst part is stuffing the envelopes and stamping.
Hope this helps